The hallmark of superconductivity is a sudden resistance drop to zero ohms and strong diamagnetism (the Meissner effect) near the same temperature. In numerous tests a small amount of the compound (Tl5Pb2)Ba2Mg2Cu9O17+ consistently produced sharp resistive transitions near 28.5 Celsius (see above graphics), and diamagnetic transitions also near 28.5C (below). The transitions were unambiguous, repeatable, and at ambient pressure, making this the first observation of true room-temperature superconductivity in a copper-oxide. Unfortunately, like the 18C superconductor discovered in March 2011, these transitions occurred in a noisy environment, suggesting the volume fraction is very low. As such, any plans for immediate commercialization will have to wait for a refining method to be developed.
This achievement came as a result of efforts to reformulate the 18C superconductor discovered in March 2011. The key to improving that material was substitution of a magnesium atom into one of the C2 copper sites (see "LIGHT" region of D223 graphic at left). By doing this the planar weight ratio (PWR) of the C2 axis increases, while the PWR of the C1 axis remains unchanged. Research since 2005 has confirmed a direct correlation between PWR and Tc.
Logically, the next evolutionary step might be to place magnesium atoms into both C2 copper sites. But, experience has shown every metallic non-copper layer must have a copper layer adjacent to it on at least one side. 2 Mg atoms in the LIGHT C2 would necessitate a copper in the LIGHT C1 to meet this requirement. However, since Cu is over 2.5 times as heavy as Mg, that would significantly lower the C1 PWR, causing a drop in Tc.
As with prior discoveries that advanced high Tc through asymmetry along the C axis, (Tl5Pb2)Ba2Mg2Cu9O17+ does not form stoichiometrically (by conventional mixing of chemicals). It must be synthesized using the layer cake method, as shown below. The prototype pellets each had roughly 25 to 30 layers. And, even using this technique, the volume fraction is low, requiring very sensitive test equipment.
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